Prime olefins, defined as ethylene and propylene and are materials that are used to make polyolefins. Specifically, ethylene and propylene are used to make polyethylene polypropylene respectively—two of the most commonly used plastics. These prime olefins are produced by taking larger hydrocarbons (e.g., C6 to C8 hydrocarbons) found in petroleum feed stream and subjecting it to steam cracking or catalytic cracking. Cracking processes break apart these larger hydrocarbons into smaller olefins including ethylene and propylene. However, the cracking process produces nitrogen and sulfur impurities that require expensive purification steps.
Another process for producing higher olefins is the process known as an oxygenate to olefin process (“OTO”) process. The OTO process takes an oxygenate feed stream and converts it to ethylene and propylene. The OTO process generally includes no sulfur and nitrogen impurities and permits the use of a methanol feed stream produced from natural gas. Consequently, alcohol, alcohol derivatives, and other oxygenates have promise as an economic non-petroleum source for prime olefin production. Nonetheless, effluent from the OTO process comprises olefins with hydrocarbon oxygenates and high levels of water vapor. Accordingly, the recovery of olefins from an OTO process involves unique technical challenges.
U.S. Pat. No. 4,499,327 discloses making olefins from methanol using any of a variety of SAPO molecular sieve catalysts. The type of reactor disclosed is a fluid bed reactor where upward flow of vapors through the catalyst causes catalyst to be fluidized and to be carried by the flow of vapor. Consequently, catalyst often becomes entrained in an olefin effluent stream leaving the reactor.
U.S. Pat. No. 4,338,475 discloses a process for converting methanol to olefins where the catalyst is separated from the effluent stream by cyclone separators (or “cyclones”) in the reactor. Then, the catalyst is returned, directly or indirectly to the reactor. However, the flow of the catalyst through the reactor, cyclone separators, and other equipment in contact with circulating catalyst, subjects the catalyst to great mechanical stresses. The stressed catalyst tends to disintegrate during the process to produce dust-like particles, commonly referred to as catalyst fines. As defined herein, catalyst fines are catalyst particles whose greatest dimension is less than 30μ. Due to their small size and weight, catalyst fines are not efficiently removed by the cyclone separators (or cyclones) within the reactor. Consequently they become entrained (or suspended) in the effluent stream from the reactor. The smaller the catalyst fines, the more difficult they are to remove by conventional processes.
Removing catalyst particles in the effluent stream is taught in U.S. Pat. No. 4,935,568 (the 568 patent). The 568 patent discloses a process for preparing hydrocarbons from an oxygenate feed stream in which catalyst fines are recovered from an effluent stream by use of cyclone separators and/or sintered metal filter systems. Catalyst particles, particularly fines that are entrained with the gaseous effluent stream, make filter systems, including sintered metal filter systems, impractical. Filter elements may quickly become blocked with catalyst particles and need to be cleaned or replaced. Expensive and sophisticated filter systems, which provide for some measure of on-line cleaning, or low efficiency filters that allow a substantial portion of solids through, are typically required to provide improvements to this problem.
One OTO system is illustrated in U.S. Pat. No. 6,121,504 (the 504 patent). In the 504 patent, an oxygenate to olefin reactor produces an effluent stream. The effluent stream passes through heat exchangers for the efficient recovery of heat. Then the effluent stream passes through a quench tower. The 504 patent provides no guidance on how to manage the catalyst particles that exit the reactor entrained with the gaseous effluent stream.
Catalyst particles and other solids that leave the reactor suspended in the effluent stream from an OTO reactor pose a particular problem in the overall OTO process. These catalyst particles sometimes fall out of the gaseous effluent stream and deposit on downstream equipment in a phenomenon known to those skilled in the art as fouling. Fouling is the accumulation of solid deposits on surfaces of the recovery train of a reactor such as an OTO reactor. In conduits, fouling is believed to significantly decrease the cross sectional area for fluid flow, increasing pressure drop through the conduit and decreasing process efficiency. On heat exchangers, fouling is believed to occur on heat transfer surfaces such as tubes or fins, which increases the thermal resistance of those surfaces and causes the heat exchanger to become less efficient. Typically, this loss of efficiency negatively impacts the operability of the OTO reactor and other downstream equipment in the recovery train. To avoid this loss of efficiency or restore operability, various equipment items typically need to be shut down and cleaned, perhaps necessitating the shutdown of the entire OTO system.
Therefore, a need exists for an effective process for operating an OTO process to reduce the accumulation of catalyst particles in the OTO recovery train, and thereby reduces fouling. The present invention satisfies these and other needs.